Hitherto, in a CVT with an auxiliary transmission, for effecting a speed change in the auxiliary transmission mechanism, a so-called coordinated speed change has been carried out wherein the continuously variable transmission mechanism (which will be referred to as a variator in the following) is speed-changed in a direction opposite to that of the auxiliary transmission mechanism, so that reduction of shift shock of the auxiliary transmission mechanism is obtained (which is disclosed in for example Patent Document-1 and Patent Document-2). In Patent Document-2, there is described a continuously variable transmission in which the speed change of an auxiliary transmission mechanism consists of four phases, viz., a preparatory phase, a torque phase, an inertia phase and an ending phase. In this technology, at the inertia phase, the speed change ratio of the variator and that of the auxiliary transmission mechanism are controlled in mutually opposite directions for carrying out the coordinated speed change.
However, even when the above-mentioned coordinated speed change is carried out, there is a case in which a driving force gap (or acceleration gap, G-drop) produced at up-shifting of the auxiliary transmission mechanism is felt by a driver due to the driving condition of the vehicle. That is, at up-shifting of the auxiliary transmission mechanism, the speed change stage for effecting the torque transmission in the torque phase is shifted from 1st speed to 2nd speed, and thus, the vehicle driving force is lowered, and at the subsequent inertia phase, the speed change ratio of the variator is shifted from High-side to Low-side thereby to restore the vehicle driving force. With this series of operation, the above-mentioned driving force gap is produced during the period from the torque phase to the inertia phase.
When the driving force gap is of a type that is clearly felt by the driver, he or she would have a sluggish feeling (G-drop feeling) in acceleration thereby failing to have an intended acceleration feeling, which causes deterioration in vehicle drive feeling. Particularly, in a vehicle driving wherein the vehicle is being accelerated from a lower speed with an accelerator pedal kept depressed slightly (constant low accelerator open degree), the sluggish feeling of the driver to the acceleration becomes remarkable. The best way for preventing the driver from having such sluggish feeling to the acceleration is to eliminate or at least minimize the driving force gap per se. However, because the driving force gap is determined by the interstage ratio between the first speed and second speed of the auxiliary transmission mechanism, it is quite difficult to eliminate or minimize the driving force gap without making a big change to hardware construction and control logic.
One of objects of the present invention is thought out in view of the above-mentioned tasks and is to provide a control device for a continuously variable transmission in which a CVT with an auxiliary transmission is so constructed as to eliminate the sluggish feeling in acceleration thereby to improve the vehicle drive feeling. It is to be noted that the object of the invention is not limited to the above-mentioned object and searching for effects that are provided by after-mentioned embodiments of the present invention and not provided by conventional technology constitutes other objects of the present invention.